Electronic apparatuses such as a computer or a video recorder typically use a magnetic disk device as a storage device. Magnetic disk devices include a magnetic disk that magnetically stores information, a magnetic head that performs data read and write from and onto the magnetic disk, and a control circuit.
Such a magnetic disk includes multiple concentric tracks each of which is partitioned into sectors. The magnetic head relatively moves on the tracks as the magnetic disk rotates, reading or writing data from and onto intended sectors. Data to be recorded on each sector of the magnetic disk includes a preamble, a sync mark, and user data in this order from the beginning. Each sector stores one or more data items having such configuration.
The preamble is data to be a reference for synchronous clock with which to read data. The sync mark is an identifier for finding the beginning of user data. The control circuit of the magnetic disk device synchronizes a read clock with a preamble read by the magnetic head to read a sync mark, and then controls the magnetic head to read user data following the sync mark. The user data is demodulated in units of code word.
To deal with sync mark errors (failures in reading sync marks) because of scratches or the like on the magnetic disk, there is a technique for recovering user data unread due to a sync mark error by a forced search. In the forced search, signal sampled values of a sector to read having a sync mark error are held in a buffer, and the buffered data is demodulated while being shifted. A successful demodulation means that the user data has been successfully read. Whether the demodulation is successful or unsuccessful is determined by parity check, for example.
However, such a conventional technique may require a long processing time for the recovery of user data, if the user data corresponding to an unread sync mark spans multiple code words, since all of the multiple code words are enforcedly searched. Thus, improvement of the technique is desired.